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Abstract:

This invention relates to a method for producing alloy and intermetallic
powders. Particularly to a method for the production of titanium based
alloy and intermetallic powders. A first metal and a second metal oxide
powder are mixed with a controlled metal/metal oxide molar ratio. This
mixture is heated, becomes self propagating and leads to formation of a
mixture of alloy liquid and a oxide solid. Pressure is applied to
separate the phases and upon cooling produces a metallic solid. FIG. 1a
shows an example of a solid crushed into a powder as produced by this
method.

Claims:

1. A method of producing an alloy including the steps of:a) pressing a
milled metal/metal oxide composite powder producing a powder compact,
andb) inserting the powder compact in an open die or an extrusion die,
and, characterised by the steps of:c) applying pressure to the powder
compact, andd) heating the powder compact in the die to the required
temperature such that exothermic reactions between the metal and metal
oxide in the powder compact is ignited, become self propagating and lead
to formation of a mixture of alloy liquid and oxide solid, ande)
continuing to apply pressure to separate phases.

2. A method of producing an alloy as claimed in claim 1, characterised by
the further step of milling the mixture to produce the metal/metal oxide
composite powder.

3. A method of producing an alloy as claimed in claim 2, characterised by
the further step of mixing a first metal and a second metal oxide powder
with a controlled metal/metal oxide molar ratio to form a mixture to be
milled.

4. A method of producing an alloy as claimed in claim 3, wherein the first
metal is aluminium.

5. A method of producing an alloy as claimed in claim 3, wherein the
second metal oxide powder is TiO.sub.2.

6. A method of producing an alloy as claimed in any one of claims 3 to 5,
wherein the first Al metal and the second TiO2 metal oxide powder
are mixed together with an Al/TiO2 molar ratio.

7. A method of producing an alloy as claimed in any one of claims 1 to 6,
wherein a reaction product is an intermetallic compound TiAl.

8. A method of producing an alloy as claimed in any one of claims 1 to 7,
wherein a reaction product is an intermetallic compound Ti3Al.

9. A method of producing an alloy as claimed in any one of claims 1 to 8,
wherein a reaction product are metallic phases Ti(Al) solution and
Al2O.sub.3.

10. A method of producing an alloy as claimed in claim 6, wherein the
Al/TiO2 molar ratio is controlled using the nominal reaction
equation:4Al+3T.sub.2.fwdarw.3Ti+2Al2O3

11. A method of producing an alloy as claimed in claim 6, wherein the
Al/TiO2 molar ratio is controlled using the nominal reaction
equation:5Al+3TiO.sub.2.fwdarw.Ti3Al+2Al2O3

12. A method of producing an alloy as claimed in claim 6, wherein the
Al/TiO2 molar ratio is controlled using the nominal reaction
equation:7Al+3TiO.sub.2.fwdarw.3TiAl+2Al2O3

13. A method of producing an alloy as claimed in any one of claims 1 to
12, wherein the mixture milled is converted into an Al/TiO2 powder
with particle sizes in the range of 0.1 μm-200 μm.

14. A method of producing an alloy as claimed in anyone of claims 1 to 13,
wherein the milled Al/TiO2 powder is pressed as per step a) into a
powder compact using a die.

15. A method of producing an alloy as claimed in claim 14, wherein the
shape and configuration of the powder compact is a cylinder of 40 mm in
diameter and 40 mm in height.

16. A method of producing an alloy as claimed in claim 14 or claim 15,
wherein the powder compact is placed in a die as per step b) and a
pressure in the range of 0.01-15 MPa is applied to the compact.

17. A method of producing an alloy as claimed in any one of claims 1 to
16, wherein the die containing the powder compact is heated as per step
d) to an elevated temperature in the range of 400.degree. C.-1300.degree.
C.

18. A method of producing an alloy substantially as herein described with
reference to and as illustrated by the accompanying micrographs and
graphs.

Description:

TECHNICAL FIELD

[0001]This invention relates to a method for producing alloy and
intermetallic products. Particularly, although not exclusively the
present invention relates to a method for the production of titanium
based alloy and intermetallic products.

BACKGROUND ART

[0002]Pure titanium is a silvery-white, lustrous metal with a low density
and good strength. Titanium alloys which are formed by combining titanium
with a small fraction of other metals can be as strong as high strength
steel, but with only 60% of its weight.

[0003]Titanium and its alloys are ideal for applications in which weight
is important, since the alloys have greater strength to weight ratio than
other metal alloys. Because of its high strength to weight ratio,
titanium and its alloys are widely used in both aerospace and
non-aerospace applications.

[0004]Aerospace applications include use in gas turbine engines in both
military and commercial aircraft (where use of titanium results in
reduced engine weight while maintaining strength). In most aircraft
engines, titanium-based alloy parts account for 20% to 30% of engine
weight.

[0005]Aerospace uses for titanium constitute the largest market for
titanium, with commercial and military aerospace applications consuming
65% of titanium mill product shipments in 1997.

[0006]Non-aerospace applications include use in specialty chemical, pulp
and paper, oil and gas, marine and consumer goods industries.

[0007]Titanium alloys can also be used to replace steel in making
automotive components, but this application has been severely limited by
the high cost of titanium alloys.

[0008]This high cost is largely a result of the expensive batch processes
that are used to recover titanium from its mineral concentrates, and the
technical difficulties associated with melting and alloying titanium.
When in molten form, titanium has an extremely high tendency to react
with surrounding materials and the atmosphere which causes difficulties
in processing titanium alloys in molten form.

[0009]The conventional titanium production process, the Kroll process,
involves the reaction of TiO2 and carbon, in the form of coke, under
chlorine gas at temperatures of 800° C. to form TiCl4 and
carbon monoxide.

[0010]The titanium chloride (TiCl4) produced by this reaction exists
as a liquid and has to be purified by distillation. The liquid is
introduced into a furnace holding a magnesium melt at 680° C. to
750° C. to facilitate the formation of magnesium chloride
(MgCl2) and pure titanium.

[0011]MgCl2 is a gas, while titanium is a solid sponge. Titanium
sponge is a porous, brittle form of titanium. Sponge is an intermediate
product used to produce titanium ingots, which in turn is used to make
slabs, billets, bars, plates, sheets, and other titanium mill products.

[0012]The sponge is purified by distillation or leaching using
hydrochloric acid. The magnesium chloride can be recycled through an
electrolysis process.

[0013]The titanium sponge that is formed by this process can be further
processed to produce commercial purity titanium or titanium alloys by
vacuum arc melting or other melting methods.

[0014]If titanium or titanium alloy powder is needed, the titanium or
titanium alloy needs to be heated to a high temperature above
1650° C. to produce titanium alloy melt and the alloy melt is
atomised into liquid droplets which in turn solidifies as powders.

[0015]The limitations of this process include its complexity and the use
of chlorine. The process involves several high temperature steps where a
high amount of energy is needed. This contributes to the high cost of
titanium and titanium alloys. The use of chlorine makes the process
environmentally unfriendly.

[0016]U.S. Pat. No. 6,264,719 discloses both a titanium alloy based
dispersion-strengthened composite and a method of manufacture of same.
This patent discloses the use of dry high-energy intensive mechanical
milling and the process of producing titanium base metal matrix
composites (MMC).

[0017]High energy mechanical milling has the effect of providing the
necessary number of small particles below the micrometer size range as
well enhancing the reactivity of different particles with one another.

[0018]While this patent has provided a method of producing titanium based
MMCs at a reduced cost, it does not disclose a method for separating out
unwanted components present within the MMC or adjusting the level of
certain components to more desirable concentrations. It would be an
advantage of the present state of the art to have some way of removing
unwanted components.

[0019]JP20019211A2 discloses the production of hydrogen-containing
titanium-aluminium alloy powder.

[0020]Sieved sponge titanium of about ≦50 mm is charged into a
furnace and is heated at 300° C. to 500° C. for one minute
to one hour in a hydrogen current under about 1 to 5 atmospheric
pressure. This sponge titanium is subjected to the hydrogen absorbing
treatment contains ≧3.5 mass % hydrogen and has a 1 to 20 mm grain
size. The sponge titanium which has been subjected to the hydrogen
absorbing treatment is charged into a vessel together with aluminium
powder, grains or pieces, and the mixture is subjected to ball milling by
using a rotary ball mill or the like. The ball milling is executed in an
atmosphere of inert gas or in a vacuum. By the milling for about 10 to
200 hours, an alloy powder in which aluminium and hydrogen are allowed to
enter into solid solution of a titanium can be obtained.

[0021]However, this process uses high cost raw materials (sieved sponge
titanium) to make the hydrogen-containing titanium rich intermetallic or
alloy powders directly. This leads to high cost for production of the
titanium intermetallic and alloy powder.

[0022]Other than the specific methods as described above, there are also
several other well established methods for producing metal and alloy
powders which can be used to produced titanium or other metal and alloy
powders. These include (a) liquid-atomisation method; (b) electrolytic
method; (c) reduction method; and (d) grinding method.

[0023]The liquid-atomisation method involves preparing a metal or alloy
melt by melting pure metals or alloys. A stream of the melt is then
broken into droplets using gas, water, centrifugal forces or other means.
The droplets subsequently solidify into fine solid particles in an inert
environment such as argon or vacuum to procured powders. The disadvantage
of this method is that titanium alloy powders produced are very expensive
because of the high cost of the starting titanium metal or alloy and high
processing cost.

[0024]The electrolytic method involves using an electrolytic cell and
suitable anode and cathode materials and electrolytes that can be
operated in such away that metals or alloys particles can be produced at
the cathode side. As an example, an extension of this method which has
been applied to producing titanium metal powder is the well documented
FFC-Cambridge process which involves de-oxidation of TiO2 powder
compact into a compact of titanium powder using the electrolytic process.

[0025]The difficulty of this method to directly produce titanium alloy or
titanium based intermetallic powders is the complexity involved in
simultaneously reducing different oxides in an electrolytic process.

[0026]The reduction method is often used to produce not so active metal or
alloy powders by reducing a chemical powder such as iron oxide (FeO,
Fe2O3 or Fe3O4) and copper oxide (CuO) by a suitable
reductant chemical such as carbon or hydrogen to produce a metal powder.
One extension of the reduction powder method is the widely reported
Armstrong process where TiCl4 gas is continuously reduced by a flow
of molten sodium to produce titanium powder. The Armstrong process is
similar to the Kroll process in terms of having to involve the use of
chlorine which is corrosive and environmentally unfriendly. It is also
difficult to use these methods to directly produce titanium alloy and
titanium based intermetallic powders because of the complexity of the
reduction process.

[0027]It would be an advantage over the present state of art to have some
method which uses low cost raw materials which can lead to production of
low cost titanium intermetallic and alloy powders and can be more easily
developed into a large scale industrial process which is more energy
efficient and environmentally friendly.

[0028]All references, including any patents or patent applications cited
in this specification are hereby incorporated by reference. No admission
is made that any reference constitutes prior art. The discussion of the
references states what their authors assert, and the applicants reserve
the right to challenge the accuracy and pertinency of the cited
documents. It will be clearly understood that, although a number of prior
art publications are referred to herein, this reference does not
constitute an admission that any of these documents form part of the
common general knowledge in the art, in New Zealand or in any other
country.

[0029]It is acknowledged that the term `comprise` may, under varying
jurisdictions, be attributed with either an exclusive or an inclusive
meaning. For the purpose of this specification, and unless otherwise
noted, the term `comprise` shall have an inclusive meaning--i.e. that it
will be taken to mean an inclusion of not only the listed components it
directly references, but also other non-specified components or elements.
This rationale will also be used when the term `comprised` or
`comprising` is used in relation to one or more steps in a method or
process.

[0030]It is an object of the present invention to address the foregoing
problems or at least to provide the public with a useful choice.

[0031]Further aspects and advantages of the present invention will become
apparent from the ensuing description which is given by way of example
only.

DISCLOSURE OF INVENTION

[0032]According to one aspect of the present invention there is provided a
method of producing an alloy including the steps of: [0033]a) pressing a
milled metal I metal oxide composite powder producing a powder compact,
and [0034]b) inserting the powder compact in an open die or an extrusion
die, and characterised by the steps of: [0035]c) applying pressure to the
powder compact, and [0036]d) heating the powder compact in the die to the
required temperature such that exothermic reactions between the metal and
metal oxide in the powder compact is ignited, become self propagating and
lead to formation of a mixture of alloy liquid and oxide solid, and
[0037]e) continuing to apply pressure to separate phases.

[0038]Preferably, the steps above may include milling the mixture to
produce the metal/metal oxide composite powder.

[0039]Preferably, the steps above may include mixing a first and a second
metal oxide powder with a controlled metal/metal oxide ratio to form a
mixture to be milled.

[0040]Preferably, pressure may be applied by pressing or extruding the
solid/liquid mixture to separate the molten intermetallic or metallic
phases of the metal rich intermetallic or metallic liquid from the solid
metal oxide phase or other ceramic phases, producing metallic or
intermetallic lumps or ingots.

[0041]Throughout the present specification the term `metal based alloy` in
accordance with the present invention should be understood to mean a
metallic material consisting of a mixture of at least two metals or of
metallic with non-metallic elements. While it should be appreciated that
there is at least two substances in a metal based alloy, there is
theoretically no limit to the number of substances that make up a metal
based alloy. This term may now be simply referred to as an alloy.

[0042]In preferred embodiments of the present invention the metal in the
metal based alloy is predominantly titanium.

[0043]However, this should not be seen as a limitation on the embodiments
envisaged for this invention. The metals that predominantly make up the
metal based alloy can include preferably nickel, platinum, aluminium,
palladium and possibly any others from the periodic table. The starting
materials can include oxides and other ceramic materials.

[0044]It should be appreciated to those skilled in the art that an
intermetallic powder is a substance that contains one or more metal
compounds divided into many small individual particles.

[0046]For ease of reference throughout the specification, TiAl and
Ti3Al will now be collectively referred to as TixAl. This term
should not be seen as limiting.

[0047]An advantage of this method is that it uses low cost raw material
such as Al and TiO2 to synthesise titanium rich metallic or
intermetallic powders directly, which can lead to the production of low
cost titanium based metallic or intermetallic powders.

[0048]Specifically, a first metal such as Al and a second metal oxide
powder such as TiO2 are mixed together with an Al/TiO2 molar
ratio which can be controlled using one of the following nominal reaction
equations:

[0051]In preferred embodiments the molar ratio of Al:TiO2 for the
production of the titanium rich intermetallic compound TiAl is 7:3.

[0052]The molar ratio of Al:TiO2 for the production of the titanium
rich intermetallic compound Ti3Al is 5:3.

[0053]A further metallic phase of Ti(Al) solution may be produced by the
Al:TiO2 molar ratio of 4:3.

[0054]The mixture is converted into an Al/TiO2 composite powder or an
Al/TiO2 powder mixture with particle sizes in a typical range of 0.1
μm-200 μm.

[0055]The powder is mixed and milled by a milling means in order to create
a powder with a high area of reaction interfaces. The milling time
typically ranges from 1 minute to 100 hours.

[0056]In preferred embodiments the milling means may be a high-energy
mechanical mill such as a ball mill or a discus mill.

[0057]This is a mechanical process in which the mixture of the metallic
powder and oxide powder is treated to alter the shape, size and
microstructure of the particles through the impact of milling balls or
discus typically made of hardened steel upon the powder particles within
a container also typically made of steel hardened steel.

[0058]In some embodiments, the milling of the powder is undertaken under
an inert environment. This could include an inert atmosphere such as
argon, or a vacuum.

[0059]The milled Al/TiO2 composite powder or powder mixture is
pressed into a powder compact of variable shape and size using a
mechanical press and a metal or ceramic die.

[0060]The term `powder compact` is a term known to someone with skill in
the art of powder metallurgy and refers to compressing a metal powder to
form a powder agglomerate suitable for sintering.

[0061]In preferred embodiments of the present invention the shape and
configuration of the powder compact may be typically a cylinder of 40 mm
in diameter and 40 mm in height.

[0062]However, this should not be seen as a limitation on the embodiments
envisaged for this invention. A number of sizes and shapes may be used to
produce the powder compact depending on the processing requirements.

[0063]Preferably, the strength of the compact should be sufficient to
allow a light pressure typically in the range of 0.01-15 MPa being
applied to the compact in an open die without causing the compact to
fracture or collapse at temperatures up to the ignition temperature of
the compact.

[0064]It is envisaged that the shape of the compact may have features such
as a centre hole and/or surface grooves which can assist the liquid
flowing out of the compact in the later stage of the process.

[0065]The powder compact is placed in either an open die or an extrusion
die, and a light pressure typically in the range of 0.01-15 MPa is
applied to the compact. An open die is known to someone with the skill in
the art of metallurgy and refers to a die configuration typically
consisting of two pieces with typically flat working surfaces. Open dies
with non-flat working surfaces may be used to assist the solid-liquid
separation in later stage of the process.

[0066]An extrusion die is also known to someone with the skill in the art
of metallurgy and refers to a die configuration typically consisting of a
piece with a cavity of controlled size and shape and an outlet opening of
controlled size and shape and a plunger. The dies may be made from heat
resistant materials such as alumina, tungsten carbide, silicon carbide,
H13 die steel or other high temperature ceramic or metallic materials.

[0067]The die containing the powder compact is heated to an elevated
temperature using a heater or a furnace under an inert atmosphere of
argon or helium or in a vacuum. This elevated temperature is typically in
the range of 400° C.-1300° C.

[0068]In preferred embodiments the die and the heater or furnace are
surrounded with insulation material such as alumina particle board to
protect loss of heat.

[0069]In preferred embodiments the powder compact is heated to the
temperature required to ignite the exothermic reactions between the metal
and oxide in the powder compact while the powder compact is being pressed
with a light pressure typically in the range of 0.01-15 MPa.

[0070]This temperature which is typically in the range of 400°
C.-1300° C. ignites the powder compact and allows an exothermic
reaction between Al and TiO2 to take place and become self
propagating. The ignition temperature of the powder compact depends on
the composition, size and microstructure of the powder particles in the
composite and the degree of powder compaction in the compact. Typically
the ignition temperature can be measured by conducting thermal analysis
of the composite powder or powder compact.

[0071]It is envisaged that the ignition of the of the compact is high
enough so that the heat generated from the self-propagating reaction is
sufficient to heat the reaction products to a temperature above the
melting point of the metallic or phases, and also allow the melt to stay
for a sufficiently long time to allow at least a substantial portion of
it to be squeezed out of the solid/liquid mixture. Typically, a higher
ignition temperature can lead to an increase of the fraction of the
liquid to be separated out from the solid/liquid mixture.

[0072]The advantages of this invention is that it generates so much heat
at a sufficiently high rate that it heats the reaction products to a
temperature which is above the melting point of titanium rich metallic or
intermetallic phases, but below the melting point of Al2O3. A
solid/liquid mixture allows the separation process of Al2O3
from the titanium rich metallic or intermetallic phases to be more
effective and less expensive.

[0073]The combustion reaction used to produce TixAl from aluminium
and titanium dioxide powders results in the formation of Al2O3
particles and a titanium rich metallic or intermetallic phase.

[0074]While Al2O3 is a desired component of a metal-ceramic
composite e.g. TixAly(O)/Al2O3, it is often desirable
to separate the Al2O3 phase in order to produce high value
titanium base metallic or intermetallic material such as Ti3AI.

[0075]Once the titanium rich intermetallic or metallic phases melt and
turns into a molten titanium alloy, the mixture of titanium alloy liquid
and Al2O3 solid is able to be separated by pressing of the
solid/liquid mixture.

[0076]In preferred embodiments the separation process may be performed by
pressing the solid/mixture using an open die or extruding the
solid/liquid mixture using an extrusion die. The pressing or extruding
action enables the molten titanium to flow easily out of the mixture.

[0077]An advantage of the die apparatus is that it allows the liquid to
flow easily out of the mixture. In this way, the titanium rich
intermetallic or metallic liquid is separated from the Al2O3
phase.

[0078]An advantage of the separation process is that the solid/liquid
separation provides a degree of purification resulting in the titanium
rich powder being pure enough for some applications.

[0079]In preferred embodiments of the present invention upon flowing out
of the solid/liquid mixture and cooling, the molten titanium alloy
solidifies and turns into titanium rich intermetallic compounds and/or
metallic phases in the ingot, granule or lump form. The initial cooling
of the molten titanium alloy occurs rapidly once it flows to a lower
temperature zone. Further cooling may be done by switching off the heat
or furnace and leaving the titanium rich ingot, granules or lumps to set,
or by using flowing argon.

[0080]The ingot, granules or lumps may be crushed into a titanium rich
powder containing less than 10% oxygen in weight. The ingot is fairly
brittle owing to this oxygen content and therefore is easily crushed.

[0081]In preferred embodiments the present invention the ingot may be
crushed by using a ball mill or a discus mill.

[0082]There are a number of advantages associated with this method. The
method allows the use of lower grade and therefore lower cost raw
materials (Al and TiO2) to make the titanium rich intermetallic or
alloy powders.

[0083]For example, TiO2 may be obtained from slag which contains
approximately 30-35 molar percent of TiO2 or enriched slag with a
TiO2 content of 80 molar percent or higher.

[0084]This leads to the production of low cost titanium alloy and
intermetallic powders.

[0085]The method utilises the formation of a solid/liquid mixture through
a controlled self-propagated exothermic reaction. This allows pressing of
the solid/liquid mixture to separate the titanium rich phase and the
ceramic phase.

[0086]The advantage of this separation process is that it allows a very
quick and easy separation of components compared to known and existing
prior art methods. The decrease in the number of steps and the ease of
same leads to lower costs for the separation procedure.

[0087]A further advantage of the separation process is that the
solid/liquid separation provides a degree of purification resulting in
the titanium rich powder being pure enough for some applications.

[0088]Yet a further advantage of the solid/liquid separation process is
that it can be used to produce alloys containing three or more metallic
alloying elements such as Ti--Al--V alloys. As an example, when the
production of such complex alloys or intermetallic compounds is desired,
the initial Al/TiO2 powder mixture or composite powder and the
corresponding powder compact needs to contain a required portion of other
metal oxide (or oxides) such as V2O5. This method allows the
metal oxide to be reduced by the metal constituent such as Al to produce
alloys containing three or more metallic alloying elements.

BRIEF DESCRIPTION OF DRAWINGS

[0089]Further aspects of the present invention will become apparent from
the following description which is given by way of example only and with
reference to the accompanying micrographs and graphs in which:

[0090]FIG. 1 (a) The microstructure of the intermetallic compound TiAl as
produced by this method; (b) Energy dispersive X-ray (EDX) spectrum from
the TiAl phase of the microstructure showing the composition of this
phase; (c) X-ray diffractometry (XRD) pattern of the intermetallic
compound TiAl as produced by this method (The small fraction of inclusion
particles (<5%) are Al2O3);

[0091]FIG. 2 (a) The microstructure of the intermetallic compound
Ti3Al as produced by this method; (b) Energy dispersive X-ray (EDX)
spectrum from the Ti3Al phase of the microstructure showing the
composition of the phase; (c) X-ray diffractometry (XRD) pattern of the
intermetallic compound TiAl as produced by this method (The small
fraction of inclusion particles (<5%) are Al2O3);

[0092]FIG. 3 (a) The microstructure of the metallic Ti(Al) solid solution
as produced by this method; (b) Energy dispersive X-ray (EDX) spectrum
from the Ti(Al) phase of the microstructure showing the composition of
the phase (The small fraction of inclusion particles (<5%) are
Al2O3.);

[0093]FIG. 4 (a) The microstructure of the metallic Ti(Al,V) alloy as
produced by this method; (b) Energy dispersive X-ray (EDX) spectrum from
the Ti(Al,V) phase of the microstructure showing the composition of the
phase (The small fraction of inclusion particles (<5%) are
Al2O3.); and

[0094]FIG. 5 Cross-sections of the particles in the intermetallic compound
TiAl powder produced by mechanical crushing of the intermetallic compound
TiAl granules produced using this method.

[0096]The Al and TiO2 powders with a controlled Al/TiO2 molar
ratio are added together into a container. The molar ratio between Al and
TiO2 can be controlled depending on the desired product according to
one of the following nominal expressions:

[0097]For producing intermetallic compound TiAl

7Al+3TiO2→3TiAl+2Al2O3 (1)

[0098]For producing intermetallic compound Ti3Al

5Al+3TiO2→Ti3Al+2Al2O3 (2)

[0099]For producing metallic phase Ti(Al) solution

4Al+3TiO2→3Ti+2Al2O3 (3)

Step 2: Milling

[0100]The mixture of Al and TiO2 powders is milled to increase the
Al/TiO2 interface area for reaction using a high-energy mechanical
mill under argon or other inert atmosphere including vacuum. The milling
time is 2-4 hours. After milling, the Al/TiO2 powder mixture is
turned into Al/TiO2 composite powder.

Step 3: Compaction

[0101]The milled Al/TiO2 composite powder is pressed into a powder
compact typically with a cylindrical shape of 40 mm in diameter and 30 mm
in height first using a H13 tool steel die and a press at a pressure of
10-50 MPa and subsequently using a cold isostatic press at a pressure of
200 MPa.

Step 4: Reaction Preparation

[0102]The powder compact is placed between two alumina plates of 5-10 mm
in thickness, and then the stack is placed between the bottom work piece
and the plunger of an open die which is made of H13 steel and controlled
at room temperature. This set-up is enclosed in a chamber which allows
the evacuation and back-fill argon. The chamber is surrounded with an
electrical heater for heating and insulation material to prevent loss of
heat. After this set-up is completed, a pressure in the range of 0.1-15
MPa is applied to the plunger of the open die and the pressure is
maintained.

Step 5 Self Propagation Reaction and Separation

[0103]The powder compact together with the open die is heated, and when
the compact is heated to a temperature in the range of 650-700° C.
for the powder compact with an Al/Ti2 molar ratio controlled by
equation (1) for producing TiAl, or a temperature in the range of
700-800° C. for the powder compact with an Al/TiOs molar ratio
controlled by Equation (2) or (3), the exothermic reaction between Al and
TiO2 in the powder compact is ignited and becomes self propagating.
Depending on the Al/TiO2 molar ratio, the reaction products are
titanium rich intermetallic compounds (e.g. TiAl, and/or Ti3Al) or
metallic phases (e.g. Ti(Al) solution) and Al2O3.

[0104]The reaction generates so much heat at a sufficiently high rate that
it heats the reaction products to a temperature which is above the
melting point of titanium rich intermetallic or metallic phases solution,
but below the melting point of Al2O3. Once the titanium rich
intermetallic or metallic phase melts it turns into a molten titanium
alloy. Since the powder compact is being pressed by the plunger, the
mixture of titanium alloy liquid and Al2O3 solid is squeezed.
The squeeze action causes part of the molten titanium alloy to flow out
of the mixture, and therefore is separated from the Al2O3
phase.

Step 7 Cooling and Crushing

[0105]Upon cooling, the molten titanium alloy solidifies and turns into
titanium rich intermetallic compounds and/or metallic phases in often
granules or ingot form. The ingot or granules are subsequently crushed
into a titanium rich intermetallic or metallic powder. Since the material
is fairly brittle due to its substantial oxygen content the crushing is
easy.

[0106]Aspects of the present invention have been described by way of
example only and it should be appreciated that modifications and
additions may be made thereto without departing from the scope of the
appended claims.